In a way you could say that it is backed by Fiat Currencies...
Think about it you can buy and sell things with bitcoins and what do people do with those bitcoins when they have them? Buy things with them ? Maybe, but the majority of them will eventually exchange it for Fiat. Because lets face it in the physical world there's not alot of places where you can use Bitcoin.
Even though i think it's backed by Fiat I personally believe imho that this is still great. Because even though Bitcoin can not replace fiat it provides so many great features for E-Commerce. Bitcoin to me is Virtual Cash and I will be very happy to use in when it stabilizes in the end because of that.